Method and means for reducing or eliminating level variations of molten metal in high amperage electrolysis cells



F illedi Jimmy: 5, 15961 2&4

INVENTORS JACQUES BOC QUE N T/N JEAN c/war B ATTORNEY Deck 3 J1Emanuel-WIN ETAL 95 METHOD: AND; MEANS FQRi REDUCING 0R5 ELIMINASTINGLEVEL VAR A'EIONS an" MOETENZ Mama IN: HIGH AMPERAGE' smc raomsrsr mamasEileidi JhIy" 5 I961 Sheets-Sheet 5* INVENTORS JACQUES BOCQUENT/N JEANG/VRV ATTORNEY United States. Patent METHGD MEANS FQR. REDUCING ORELHWI- VARIATIONS 0F MGL'I'EN METAL, IN HIGH AMPERAGE ELECTROLYSE NATINGLEVEL The present application is a continuation-in-part of ourapplication Serial No. 641,530, filed February 21, 1957,

' now abandoned.

In the manufacture of aluminum, there are used more and more frequentlyelectrolysis cells operating with 100,- 000 amperes and higher, in whichthe elfect of the magnetic fields produce level variations of the uppersurface of the liquid metal which lies on the bottom of the crucibles ofthese cells.

These level variations may be static, the upper surface of the metalremaining motionless. They mayalso result from the more or less rapidmovements of the metal, which lowers appreciably the efliciency of theelectrolysis.

Accordingly, it is an important object of the present invention toprovide a method and means for reducingas far as possible thesevariations in the level oftheliquid metal, or even to suppress themaltogether.

High amperage electrolysis cells generally have a rectangular elongatedshape, one of the sides of the rectangle being appreciably longer thanthe other. In the following description, the magnetic field and thecurrent density at any point of the liquid metal will be defined bytheir projections on three coordinate axes starting from the same origin0, the central point of the bottom of the crucible of the electrolysiscell. The Ox axis is parallel to the longer side of the electrolysiscell; the 0y. axis lies in the same horizontal plane as the Ox axis andis perpendicular to the latter (hence, parallel to the smaller side ofthe rectangle). The Oz axis is vertical and, hence, perpendicular to the.xOy plane.

Let B denote the value of the magnetic field at a given point, and Bx,By, Hz the projections of B on the Ox, 0y and Oz axes. respestively; I

Let I denote the value of the current density, and Ix, 1y, 12, theprojections of J on the Ox, Oy and vOz axes respectively.

The static level variations of the upper surface of the liquid metal areproportional to, the products of the vertical component of the field. bythe horizontal components of the, current density, i.e.

It has been proposed to suppress the horizonal longitudinal currents I xby judiciously dimensioning (calculating) the conductors which connectone cell to the succeeding cell so as to distribute the current properlyin the bottom of the crucible. This arrangement-which has been proposedby others and is not claimed hereis elfective.

It has likewise been suggested by others, and, hence, is not the subjectof claims herein, to reduce. or suppress the transverse currents Jy bymaintaining over the entireperiphery of the liquid metal sloping massesof solidified fluorides so as to obtain a cathodic surfacesubstantiallyequal to the anodic surface. Good electrolysis efliciencies are obtainedin this manner, even though magnetic fields still exist. "However, themore the magnetic fields are reduced, the better and the more stableisthe. operation of "ice the electrolysis cell, should the current happento spread out accidentally, that is to say when the Jy component is notzero.

It is heretofore been proposed by others-and is not the subject ofclaims herein-to reduce the magnitude of the magnetic field B bydividing the conductors outside the cell into as large a number aspossible of smaller conductors, arranged with their upper endssimulatingsheets and spaced from the metal. A marked improvement is achieved inthis manner, but these arrangements interfere sometimes with the Work ofthe operators.

Finally, it has been proposed to suppress the magnetic fields by placingthe conductors far away from the liquid metal and by placing, side byside, conductors through which flow currents in opposite directions.Unfortunately, this suggestionmade by others and not claimedherein--requires a large investment in conductors and leads to excessivepower losses.

The present invention, which is. based upon applicants researches, makesit possible to avoid the above disadvantages while attaining asatisfactory stable operation of high amperage electrolysis cells evenwhen they are arranged lengthwise, that is. to say, when the long sidesof the rectangle of one cellare placed in the prolongation of the longsides of the rectangle of the preceding and of the succeeding cells inthe direction of the current flow.

The invention, its advantages and the manner of its ap' plication, willbe described with reference to the annexed schematic drawings whcihrelate to an improved system for the igneous electrolysis of a moltenbath of metallic compounds to produce molten metal in a cell comprisinga crucible for containing the molten metal, an anode verticallysupported above said crucible, a cathode, a feeder system for supplyingcurrent to the anode, current outlet means associated with said cathode,and conductor means connecting the current outlet means of the cell tothe feeder system of asucceeding adjacent cell.

In the drawings:'

FIGURE 1 is a perspective diagrammatic view showing' the volume ofliquid metal lying in the bottom of the crucible of the electrolysiscell;

FIGURE 2 shows the lengthwise arrangement of a line of' cellssuccessively traversed by a current of the same intensity, thedirection. of the current being indicated by an arrow;

FIGURE 3 is a view in vertical section and perpendicular to its greatestlength of the cell crucible;

FIGURE 4 is a plan view of the cell of FIGURE 3;

FIGURE 5 is a view in elevation of two successive cells, in thedirection of their greatest length, the current being supplied to thecells at one end only;

FIGURE 6 is a view similar to that of FIGURE 5, but with a modifiedcurrent supply, and

FIGURE 7 shows a sectional view of the electrolysis cell as in FIGURE 3,together with curves, the intersection of which enables determination ofthe optimum position of. the conductors according to the presentinvention, as will be described below.

FIGURE 8 isalso a view in vertical section and perpendicular toits-greatest length of a cell in a lengthwise arrangement of a line ofcells (as illustrated in FIGURE 2) wherein the conductors are showndisposed as they were according to an actualexample of the prior artpractice, while FIGURE 9 is a similarview of the same cell, but with theconductors disposed in accordance with the teachings ofthe presentinvention. a

In the various figures, the same or equivalent parts are ceding-cell anddistributes. it to. the anode system of the cell. 3, 3 represent theconductors in their means position disposed parallel to the greatestlength of the cell. 3, 3' (FIG. 4) represent the ends of the conductorwhich collect the current leaving the preceding cell; they areelectrically connected to the feeder system 2, 2 which, in the case ofFIG. 4, is supplied with current at one end only. This is also true inthe case of the two successive cells shown in FIGURE 5, where the feeder2 is supplied at one end only.

- In contrast, FIGURE 6 shows an arrangement of two cells similar tothose of FIGURE 5, but wherein the feeder system 2 is supplied withcurrent at its both ends. The current leaving the preceding cell andbrought by conductor 3 is divided in this case into two currents, and aconductor 4-parallel to 3-connects with the other end of feeder system2. With such an arrangement, the conductors 3 in FIGURES 3 and 4 wouldrepresent the mean position of the system of conductors 3 and 4 ofFIGURE 6. These conductors 3 and 4 can be arranged as a plurality ofconductors of smaller size disposed to simulate sheets, as previouslyproposed.

The lengthwise arrangement of the electrolysis cells is adopted veryoften because it saves space and facilitates the operation; however, themagnetic eifects may be particularly large.

The method which is the object of the present invention consists innullifying the magnetic effects at the center of the crucible of theelectrolysis cell. While these effects will still exist on the rest ofthe cell surface, they will be very weak in the neighborhood of thecenter 0 and, in practice, their magnitude shows a certain symmetry withreference to that point, which insures sufiicient stability in theoperation of the electrolysis cell.

The vertical component Bz of the field produced by the horizontalconductors situated near the cell is reduced to Zero by reason ofsymmetry.

The transverse component Jy of the current density is likewise reducedto zero at the center of the cell for reasons of symmetry, and so is Ix;hence, at that point, there are no magnetic effects of static origin.

The effects of dynamic origin depend on the value of a rotational vectorR which can be calculated by means of the values of the components Bx,By, Hz and of the current densities Ix, Jy, Jz and their partialderivatives.

The mathematical expressions for the projections of the vector R on theOx, Oy and Oz axes are as follows:

..as Jx=Jy=O. The derivative variation of the vertical component of thefield along the 4 height of the liquid metal, is also zero.

dJz y variation of the vertical component of the current density alongthe transverse direction, is zero, Jz being practically constant over avery wide zone about the center 0.

Hence, at the center 0 of the cell, the components Rx and Rz of therotational vector are zero and there re mains:

The derivative dJy y dBy Ry=By Therefore, rotational movement of themetal about an axis parallel to 0y may occur at the center of the cell.To prevent this, Ry must be reduced to zero. However, 12, the verticalcomponent of the current density, cannot be reduced to zero, neither canwhich expresses the variation of this transverse component of the fieldalong the vertical.

In order to compute the component By, in FIGURE 3 let a be the distancebetween the feeder 2 and the Oz axis;

b the vertical distance of the line passing through the middle (mean)points of conductors 2 to the center of the cell;

c the distance between conductor 3 and the Oz axis; and

d the vertical distance between 0 and a line passing through the middle(mean) points of conductors 3.

a1 will designate the intensity of the current supplied to feeder 2 atthe side of the preceding cell (FIG. 4), and (1a)l theintensity of thecurrent supplied to feeder 2 from the opposite side (FIG. 6). In thecase of FIG- URE 5, 11:1.

The field produced at O by the current flowing through conductors 2 and3 is calculated in the same way as if it were the case of conductors ofunlimited (indefinite) length carrying the currents which actually flowthrough the conductors in the YOZ plane. Experience proves that there isobtained an approximation which is sufficient for practical purposes. I

The following results are obtained:

Conductors 2,2 Conductors 3,3

131! V 2I(a}) 2I(ad dBy (h -a 2+ 2 z 2+ 2 2 To reduce simultaneously tozero By and dBy it is necessary to have the following two conditions:

Equation 2 The term on can be eliminated from the two equations.

which can replace the second of the above equations.

Given a and b, i.e. the position of the feeders 2-2 which is generallydetermined by the requirements of construction and operation of thecell, it will be seen that the third equation represents a cubic curveindependent of the current distribution a in the feeders.

The first equation represents a circle the radius of which varies witha. The distance a is generally small; if we assume that it is zero,then, we obtain conditions as illustrated in FIGURE 7. Here, 5 is thecubic curve on which the conductors 3, 3 must always be placed; 6 is thecircle corresponding to oe=1, ie the situation where current is.supplied at one side only, with circle 6 having a radius of b/Z; 7 isthe circle corresponding to oc= /3 and a radius of 512/2. The meanposition of conductors 3 will then be at 8 in the case where the feedersystem is supplied with current at one end only, and at 9 when thefeeder system is supplied with current at both ends, with We of thecurrent entering at the end nearest the preceding cell, /3 by the otherend.

The position of the conductors is then determined by the followingtable:

and

all: (1/!) It has been assumed that the height b of the feeder 2 abovethe bottom of the crucible is known. This is the case most often; on theother hand, it is possible to assume that one of the dimensions 0 or dis known and the above equations can then be solved without departingfrom the scope of the invention.

The above defined position for the side conductors represents the meanpoint of the conductors which can be divided into as large a number aspossible of parallel conductors arranged in a manner simulating sheetsabout this mean point.

The examples given below, which are not given by way of limitation, Willenable a better understanding of the advantages resulting from theapplication of the present invention.

Example 1 A 100,000 ampereelectrolysis cell, arranged lengthwise and thefeeder 2 of which was supplied with current at both ends (FIG. 6),presented a bad distribution of the magnetic fields, namely, a weakfield at the side of the preceding cell, a strong field at the centerand very strong field at the side of the succeeding cell. The liquidmetal on the bottom of the cells crucible was subjected to rota siteside (the side of the next cell). By disposing the conductors 3, 3according to the method of the present invention, the magnetic efiects'beperiphery ofthe crucible.

much more stable andthere is obtained a reduction of the energy consumedamounting to 800 kwh. per ton come zero at the center 0 of the cell.They are substantially symmetrical with reference to the, point 0. Thevariations of level of the upper part of the liquid metal practicallydisappear and it is possible to maintain regular inclined masses ofsolidified fluorides over theentire The operation of the cell is Example2 A $0,000 ampere electrolysis; cell, equipped with-prebaked anodes,arranged lengthwise andhaving feeders 2, 2 supplied with current on oneside only (FIG. 5), and with the conductors 3, 3 placed on a. level withthe bottom of the crucible (d=0r), was subjected to. magnetic fieldshaving a very large horizontal'component- By at the side of thepreceding cell, a component. half this value in the vicinity of pointOwhich is reduced to zero onthe opposite side but having, in contrast, alarge vertical component Bz.

The celloperated in a very unstable manner and all the phenomenadescribedin Example: I were observed. By changing theposition of the.conductors 3', 3 in accordance with this invention to point 8 on FIG. 7,an improvement similar to the one already described. was obtained,together with a more-stable operation. Moreover, the efiiciency of theelectrolysis was increased by more than 3%, changing from 84 to 87% ofthe theoretical efiiciency.

It should be mentioned that in the practical design of a cell system,applicants. generally first select convenient values of a and b and thatis, the position of the feeders or bus-bars (monture"), these valuesbeing governed by practical considerations, such as ready access, etc.The

; values of c and d are then calculated, i-.e. determined for tors inone cell of a line of Soderberg cells arranged lengthwise which were inactual operation before the present invention. .FIGURE. 9 represents.the same cell but with the conductors disposed in accordance with theteachings of the present invention. In both instances, the cell operatedwith a current of 90,000 amperes. The cell of FIGURE 8 had the followingcharacteristics:

The feeder system (monture) 2, 2 was supplied at one end only from thepreceding cell, hence a the mean distance between the feeder 2 and theOz axis, was 0.50 m.;

b, the vertical distance of the line passing through the middle (mean)points of conductors 2, 2, to the center of the cell, was 2.60 m.; 1

c, the mean distance between the Oz axis and the conductor 3 collectingthe current from the cathode was d, the vertical distance between thecenter of the cell and a line through the middle (mean) points ofconductors 3, 3, was zero, that is to say, the conductor 3, v3 werepositioned on the 0y axis.

The cells sufiered. from severe magnetic effects: There took place anunlevelling, or level deformation, of the separating surface between thebath and the metal of the order of 10 ems, this surface rising in thedirection of 7 the current flow in the cells, i.e. rising in thedirection from A to B (cf. FIG. 2). Further, there were observedsystematic temperature differences of the bath between the two ends ofthe cell, the zone where the level of the metal was highestbeing thecoldest.

Additionally, the shape of the solidified lateral inclined mass (talus)of fluorides of the bath was asymmetrical. It was quite pronounced inthe zone (region) B (FIG. 2) where the height of the metal was a maximumand where, also, several centimeters of mud appeared on the bottom ofthe crucible. An inclined frozen mass (talus) was practicallynon-existent on the opposite side A where the bath and the metalappeared to be in continuous agitation. At the same time, the operationof these cells were unstable and inefiicient; the operating results wereinferior to those of smaller cells; the average voltage was higher (byabout 0.15 volt) and the yield of the current smaller (by about 5%).Actually, the yield of the current (Faraday yield) was 82% in lieu of87%.

The cell of FIGURE 8 was subsequently modified in accordance with theteachings of the present invention in the manner illustrated in FIGURE9. That is, the feeders 2, 2 were left in the same position but weresupplied with current at both ends. FIGURE 9 shows the cubic curve 5satisfying the equation given earlier in the specification, and furthershows the various circles corresponding re spectively to the value of =1100-0 11:0.9 90-10 =01; (80-20) =07 70-30 and (1:05 (60-40) As will beseen from FIGURE 9, the most advantageous position of the conductors 3,3 is that corresponding to 06:0.8 (8020), because the conductors are notunder the sub-structures of the cell and, yet, are not too far awaytherefrom. The conductors 3, 3 are those which collect the currentleaving the cathode, and 4 is the conductor which supplies the 20% ofthe amperes to the end B of the feeder system 2, 2. H

As shown on FIGURE 9, the point of intersection of the circle 8020 wdiththe cubic 5 is the mean point of the conductor assembly 3, 3 and 4. Thecoordinates of the conductors are =0.50 m. b=2.60 m. c=2.80 m. d=1.56 m.

When the cell was modified in the manner just described, the detrimentaletfects of magnetic origin disappeared in the cell: the upper surface ofthe liquid metal became substantially horizontal, the solidifiedinclined masses (talus) of the bath were uniformly shaped and easy tomaintain. Moreover, the energy consumption (kw./hour) per ton ofaluminum was reduced from 16,750 kw./h. to 15,250 kw./h. per ton, thatis to say, a gain of 1500 kw. per hour or 9% In the appended claims, theseveral symbols have the meaning given to them in the presentspecification.

We claim:

1. An improved system for the igneous electrolysis of a molten bath ofmetallic compounds to produce molten metal, and wherein detrimentaldeformation of the level of the molten metal is substantially inhibited,comprising in combination: a plurality of successively lengthwisedisposed cells each including a crucible for containing the moltenmetal, an anode vertically supported above said crucible, a cathode; asystem of feeders for supplying current to each said anode, said feedersbeing spaced laterally from the center of the crucible by a distance aand vertically above said crucible by a distance b, the distance aalways being smaller than b, said distances a and b being determinedarbitrarily; current outlet means associated with each said cathode;conductor means connecting the current outlet means of each cell to thefeeders of the succeeding adjacent cell; said conductor means beingspaced laterally from the center of the crucible by the distance c andvertically below and relative to the center of the crucible by thedistance d, said conductor means being located at the point ofintersection of a cubic curve passing through the center of the crucibleand satisfying the equation and a circle, likewise passing through saidcenter and in which a designates the proportion of the intensity of thecurrent supplied to the feeders at the end nearest the preceding cell,on being 1 when all of the current is supplied to the end of the cellnearest the preceding cell.

2. A system according to claim 1, wherein the conductor means comprisesa number of parallel conductors, and the lateral distance c equals themean position of the parallel conductors from the center.

References Cited in the file of this patent UNITED STATES PATENTS2,761,830 Kibby Sept. 4, 1956 FOREIGN PATENTS 740,025 Great Britain Nov.9, 1955 740,063 Great Britain Nov. 9, 1955 167,946 Australia July 12,1956

1. AN IMPROVED SYSTEM FOR THE IGNEOUS ELECTROLYSIS OF A MOLTEN BATH OFMETALLIC COMPOUNDS TO PRODUCE MOLTEN METAL, AND WHEREIN DETRIMENTALDEFORMATION FO THE LEVEL OF THE MOLTEN METAL IS SUBSTANTIALLY INHIBITED,COMPRISING IN COMBINATION: A PLURALITY OF SUCCESSIVELY LENGTHWISEDISPOSED CELLS EACH INCLUDING A CRUCIBLE FOR CONTAINING THE MOLTENMETAL, AN ANODE VERTICALLY SUPPORTED ABOVE SAID CRUCIBLE, A CATHODE; ASYSTEM OF FEEDERS FOR SUPPLYING CURRENT TO EACH SAID ANODE, SAID FEEDERSBEING SPACED LATERALLY FROM THE CENTER OF THE CRUCIBLE BY A DISTANCE AAND VERTICALLY ABOVE SAID CRUCIBLE BY A DISTANCE B, THE DISTANCE AALWAYS BEING SMALLER THAN B, SAID DISTANCES A AND B BEING DETERMINEDARBITRARILY; CURRENT OUTLET MEANS ASSOCIATED WITH EACH SAID CATHODE;CONDUCTOR MEANS CONNECTING THE CURRENT OUTLET MEANS OF EACH CELL TO THEFEEDERS OF THE SUCCEEDING ADJACENT CELL; SAID CONDUCTOR MEANS BEINGSPACED LATERALLY FROM THE CENTER OF THE CRUCIBLE BY THE DISTANCE C ANDVERTICALLY BELOW AND RELATIVE TO THE CENTER OF THE CRUCIBLE BY THEDISTANCE D, SAID CONDUCTOR MEANS BEING LOCATED AT THE POINT OFINTERSECTION OF A CUBIC CURVE PASSING THROUGH THE CENTER OF THE CRUCIBLEAND SATISFYING THE EQUATION